A conventional ignition plug for an internal-combustion engine such as a car engine uses an electrode tip made of precious metals such as platinum for an end portion of an electrode in order to increase resistance to spark consumption. However, since the precious metals are very expensive and generally used for luxury cars, iridium Ir is generally used for low cost cars.
However, there is a problem in that the iridium is easily oxidized and vaporized at a high temperature of 900 to 1000° C. Therefore, when the iridium is directly used for a spark portion of the electrode, the iridium may be rapidly consumed by oxidation and vaporization. Accordingly, although the ignition plug using the iridium for the spark portion of the electrode has high durability in a low-temperature condition such as in city road driving conditions, the durability of the ignition plug significantly decreases in high speed driving conditions.
Specifically, the iridium that is a main element of the electrode tip is combined with oxygen when oxidized. The generated iridium oxide IrO2 has non-volatile property and corrosion resistance. However, as a temperature increases (to about 900° C.), volatile iridium oxide IrO3 is generated. A temperature in a cylinder normally increases to about 100° C. and sometimes increases to about 2000° C., so that the iridium oxide IrO3 having volatility is mainly generated. In order to cover the iridium oxide IrO3 having low corrosion resistance, rhodium Rh is widely used. When an iridium-rhodium Ir—Rh alloy is oxidized at a high temperature, rhodium oxide RhO2 is generated at a surface of the alloy and covers a surface of the electrode tip, and this prevents the iridium oxide IrO3 from volatilizing. Accordingly, the electrode tip can be prevented from being rapidly consumed at a high temperature.
However, the rhodium Rh is also expensive, so that an alloy which is cheap and has a similar performance is needed.